The successful functioning of a liquid crystal device depends on the ability of the liquid crystal molecules within that device to adopt and maintain an imposed alignment. Alignment of the liquid crystal molecules is achieved by use of an orientation layer which defines a direction of orientation for the liquid crystal molecules of the device with the result that the molecules become aligned with the direction of orientation defined by the orientation layer. In addition to this directional alignment, the orientation layer is also able to impart to the liquid crystal molecules an angle of tilt so that the molecules align themselves at an angle to the surface of the orientation layer rather than lying parallel thereto.
Methods of preparing orientation layers for liquid crystal materials are well known to a skilled person. Customarily used uniaxially rubbed polymer orientation layers such as, for example, polyimides however impact a series of disadvantages like dust generation during the rubbing process and destruction of the thin film transistors. Scratches due to brushing are another problem, which becomes particularly evident when the pixels are of the order of 10 micrometers or less as for instance in microdisplays. Furthermore, the rubbing process does not allow the production of structured layers. Alternatively, orientation layers in which the direction of orientation can be predetermined by irradiation with polarized light avoid the problems inherent to the rubbing process. In addition, it is possible to provide areas having different orientation and tilt and thus to structure the orientation layer as described for example in Jpn. J. Appl. Phys., 31(1992), 2155-2164 (Schadt et al.).
Photoactive materials for the orientation layers were also described for example in WO-A-99/49360 (Rolic AG), JP-A-10-195296, JP-A-10-232400 (both Samsung Electron Devices Co., Ltd.), WO-A-99/15576 (Rolic AG) and WO-A-99/51662 (Kanegafuchi Kagaku Kogyo KK). Orientation layers comprising materials described in these publications, however, have the disadvantage that for certain applications, particularly when used in LCoS liquid crystal displays, a long exposure to light of high intensity may have a degradation effect, which in turn may lead to an undesirable reduction of the resistivity or “holding ratio” of the adjacent liquid crystal mixture. Low holding ratio values mean changes in brightness and contrast over time and thus a disadvantageous deterioration of the viewing characteristics, such as unstable graduations of the grey tones, of a display.